The present invention relates generally to prosthetic implant devices and pertains, more specifically, to a femoral prosthesis of the type having a stem to be affixed within the femur for providing a ball at the proximal femur, with the stem extending longitudinally downwardly into the femur and terminating at a remote distal end of the stem.
Ordinarily, the implant of a femoral prosthetic device is accomplished by inserting the stem of the prosthesis longitudinally into a cavity formed in the prepared femur and affixing the stem, along at least a portion of the length thereof, within the cavity through the use of an interference fit or cement. In this manner, the stem of the device is coupled directly to the bone of the femur for securement and for the transmission of loads through the device to the femur. Because of the length of the stem, securement and load transmission are spread over a concomitant length of the femur, thereby reducing unit loads on the bone of the femur. Experience has shown that certain components of the forces exerted upon the prosthesis during use are transferred to distal portions of the stem, rather than being accommodated by proximal portions, with the result that higher stresses are present at corresponding nonproximal portions of the femur, while stresses at the proximal portions of the femur are reduced. For example, transverse forces on the ball of the prosthesis will tend to swing the prosthesis about the proximal end of the stem, thereby exerting bending stresses on the stem and establishing forces tending to move the distal end of the stem axially relative to the femur, as well as in a transverse direction. The affixation of the distal end of the stem to the femur resists such movement and stresses become concentrated at the affixed distal end. Excessive stress at that location can result in pain and can cause damage to the femur in the vicinity of the distal end of the stem of the prosthesis. In addition, where load is transferred away from the proximal end of the femur, a tendency arises for atrophy to take place at the proximal end, with a resultant loosening of the affixation of the proximal end of the stem, requiring early replacement of the entire prosthesis. The condition where stress tends to be concentrated at distal portions of the stem and reduced at proximal portions sometimes is referred to as stress shielding. Stress shielding is an undesirable condition which reduces or removes stresses from the proximal portions of the stem, which proximal portions are best able to accommodate the stress.
It has been suggested that stress shielding can be reduced, and even eliminated, through the employment of a sheath of synthetic resin material which receives the metallic stem of the prosthesis and is affixed within the femur such that the distal portion of the metallic stem can slide axially along the sheath in response to bending loads. As described in U.S. Pat. No. 4,619,659, the synthetic resin sheath is permanently implanted within the femur, and the metallic stem is releasably fitted into the sheath.